LEDs are known to act as a source of emitted light for a wide variety of applications. LEDs are known to provide many advantages over incandescent and fluorescent illumination because of their long operating life, high efficiency, lightweight, and low profile.
FIG. 4 is a schematic diagram illustrating a conventional LED display including an LED driver circuit 50 for driving an LED chain 52 made up of serial connected LEDs 53-1 to 53-N. LED driver circuit 50 includes a DC-DC boost converter 51 that is utilized in conjunction with a passive inductor LEXT and a passive discharge capacitor CEXT. DC-DC boost converter 51 includes a comparator (operational amplifier) 55, a pulse width modulator (“PWM”) 58, an internal capacitor CINT, an npn bipolar transistor T1 and a zener diode S1. An external voltage (VIN) is supplied, for example, from a battery, to the anode of diode S1 and to the collector of transistor T1 by way of inductor LEXT. The voltage level at the output node 54, which is connected to a first end of the LED chain 52, is established by a user supplied reference voltage “VREF” applied to the input terminal (node 57) that is connected to the inverting input terminal of comparator 105, also sometimes referred to in the art as the “error amplifier”. The second end of LED chain 52 is connected to the non-inverting input terminal of comparator 55, which is also connected to ground by way of an appropriately sized resistor “REXT” 56. The output voltage at node 54 adjusts until the loop through boost converter 51 controls the current passing through LED chain 52 such that the current is defined as I=VREF/REXT, where VREF is a regulated voltage powered from VIN. Light output from LED chain 52 is proportional to the current generated by LED driver 50, and can therefore be selectively increased by way of increasing the current generated by LED driver circuit 50. LED driver circuits similar to those shown in FIG. 4 are used in commercial products such as the Model 2287 integrated circuit manufactured by the assignee of the present application, that drive LED chains such that current feedback substantially constantly adjusts the power to the LED chain (see also, e.g., Min et al., U.S. Pat. No. 6,586,890, which is incorporated herein by reference in its entirety).
LEDs have improved in light emitting efficiency (i.e., conversion of electricity to light) by several orders of magnitude over the past decade. Newer LEDs provide an advantage over early LEDs in that, when provided sufficient power, they emit enough light to be seen in direct daylight. In contrast, early LEDs (i.e., those produced in the mid to late 1990s) appeared to be OFF when operated in direct daylight, no matter what level of current was applied to the LED. The efficiency improvement of newer LEDs has made possible their use in efficient outdoor video billboards.
However, with the increased light emitting efficiency of current LEDs, a new problem arises in that they can emit sufficient light to be visible with only a few microamps of current flowing through them. This low on-current can have negative repercussions in real life applications if the driver circuit driving the LEDs (e.g., driver circuit 50, described above with reference to FIG. 4) has any significant level of leakage current. For example, in stadium video displays or video billboards, when a pixel is intended to be off, small leakage currents and/or capacitive discharging can cause the pixel to continue glowing. These video displays typically turn the LEDs (pixels) ON-OFF at very high rates (PWM) to achieve an apparent variation in brightness. Leakage current and/or capacitive discharging becomes a problem in night viewing, for example, when one pixel is driven at a low brightness level (i.e., the pixel is “sort of OFF”) and an adjacent pixel is turned “completely OFF”. In this case, capacitive discharging can cause the “completely OFF” pixel to continue to glow for an undesirable period after the drive voltage is terminated, and the leakage current can cause the completely OFF pixel to appear to be lit at all times. That is, the light contrast between sort-of OFF and completely OFF pixels is reduced or completely lost due to capacitive discharging and leakage current. Another example involves emergency lighting, where leakage currents can cause LEDs to glow enough that in low light situations the emergency lights may appear to be ON.
The leakage/emission problem summarized above most affects LED drivers designed around LDO and switching regulator topologies, but can be can also be found in linear or DC drive topologies as well. In these products small leakage currents passing through the switching or control transistor are considered to be inconsequential and in many cases may be unavoidable due to the characteristics of the semiconductor process and the applied voltages. One solution to the low on-current characteristic of LEDs would be to produce LED driver circuits that do not generate any appreciable leakage current. However, this goal would require special semiconductor processes or device designs that would increase production costs over LED driver circuits designed and produced using conventional processing methods.
Further, even if special fabrication processes were used to generate a “perfect” LED driver circuit (i.e., an LED driver circuit exhibiting zero leakage current), undesirable current may still be caused, for example, by impurities on the PC board supporting the LED chain. That is, even if a perfect, non-leaking driver IC is produced, if the user's assembly process leaves residue on the display board that allows a leakage current to flow through the LEDS, the LEDs can appear to be turned on when they are intended to be turned off.
What is needed is a LED driver circuit that avoids the current/emission problems associated with conventional LED driver circuits.